Heat shrinkable tubing is a thermopolymer product that shrinks in all directions when heated. The effect is used in the technique for isolating soldered, detachable and other electrical connections.
The history of the invention of heat shrinkable tubes
The heat shrinkable tube is made of polymers that can reversibly turn into a liquid or viscous state under the action of temperature. These are mainly polyolefins:
- Polyvinyl chloride( halogenated polyolefins).
And other materials included in the group of thermoplastics. Polyolefins are considered as chain polymers of structural purpose. A characteristic lack of literature on the subject, although PVC is readily discussed as a basic solution for plastic windows, environmental impact studies are being conducted. But about shrink materials book can not be found on the Internet.
It is known that in 1962 — specifically on July 23 — Judson Douglas Wetmore, an engineer from Rachem, invented a heat shrink tube as part of a third-party study. Three years later, US3396460 A was declared and probably receives a share from each manufactured unit. The inventor has positioned his own offspring as a method for combining polymer structures. He wrote that when heated, the tube melts and tightly covers the part inserted inside.
Judson claims that he was inspired by an invention dated 1936( US2027962 A).It relates entirely to thermoplastics. The author has invented a new method of production using substances that, when heated, change shape easily. And in a wide range of temperatures, which simplifies the process of manufacturing parts. The invention is closely connected with the tests developed by the organization ASTM - it was about thermoplastics.
Let's return to Judson. The production process of the heat shrink tube begins with the choice of material. A suitable polymer is chosen, for example, neoprene. In the process of heating, additives are added there according to the future use of the material. Then comes the process of formation, recognized as the key. The polymer tube is placed in a vacuum where it is heated. Usually due to infrared waves. As a result, the product is stretched in all directions.
When the desired diameter is reached, a sharp cooling follows. In a vacuum happens quickly. It turns out, the polymer solidifies in a highly stretched state. When lightly heated - compressed. This is called a shrink tube in production.
On August 30, 1978, US patent No. 4,188,443, in the title containing the notion of shrinkable film, was filed. And here we are talking about thermoplastics. Inventors describe the component:
- The film consists of five polymer layers.
- Central( third) consists of polyester or copolyester.
- It is surrounded by a( second and fourth) ethylene-vinyl acetate copolymer.
- The shell is an ethylene-propylene copolymer.
Material is positioned as a packaging. Today, on YouTube, they show how control panels are put on the film to protect them from the effects of dirty hands. As a result, the device acquires protection from moisture and is less oxidized by air. The meaning of the presence of the mass of the layers is that the polyolefins are characterized by extreme shrinkage properties. Up to four times more compressed than PVC used in the food industry. To bring the properties of the product to the usual packaging that is used on existing equipment, and it took a few layers.
There are many thermoplastics, the qualities are different. Most of the final materials are supplied in a small amount with additional modifiers to impart specific properties. A short list of such additives:
In contrast to cured thermoset plastics and cured elastomers, thermoplastics become viscous reversibly. That contributes to the simplification of obtaining the desired shape of the product and the molecular lattice. Examples of technological methods: extrusion, casting, stamping, vacuum molding, welding. Thermoplastics are usually divided:
- Molecular structure:
- Carbon chain: polystyrenes, polyacrylates, copolymers, polyolefins. Synthesized along the radical-chain or ion-chain path.
- Hetero type: polyacetals, polyesters. Synthesized by ionic polymerization of cyclic or polycondensation of bifunctional monomers.
- Physical structure:
- Amorphous, with rigid molecules( I).The degree of crystallinity does not exceed 25%.Bright representatives are polystyrene, polyvinyl chloride and other chain-chain polymers with irregular structure. Polyamides, polyesters and polyethers and other hetero-chain polymers. Stamping and extrusion( extrusion) are performed at the glass transition temperature, molding - at the temperature of fluidity.
- Crystal medium degree( II).The glass transition temperature is close to room temperature. Pentaplast, polytrifluorochloroethylene, polymethylpentene are recognized as prominent representatives. Molding is performed at a temperature above melting.
- Crystal high degree( III).The glass transition temperature of the amorphous form is below room temperature. Under normal conditions exhibit plasticity. Below the glass transition temperature become brittle. Properties are determined by the degree of crystallinity. Bright representatives became polyethylene and polypropylene. Casting and extrusion are carried out at the melting temperature, punching - near this value.
Mechanical properties of thermoplastics
Mechanical properties are expressed in plasticity, strength, the dependence of the deformation result on the rate of application of force, temperature and other factors. It is customary to single out indicators characterizing the material in terms of resistance to external forces:
- Destructive stress:
- When stretched, it varies from 1.2 to 12 kgf / sq.mmThe prevailing rates of phenylone.
- When compressed, it varies from 0.5 to 12 kgf / sq.mmThe highest rates of polycarbonate.
- When bending, varies from 1.2 to 14 kgf / sq.mmThe superior performance of polyamide-6.
- Tensile yield strength varies from 0.75 to 8.5 gks / sq.mmThe best performance in polyamide-6.
- Elongation at break, varies from 1.5 to 800%.The prevailing indicators are high density polyethylene and polypropylene.
Many theories have been developed regarding the destruction of thermoplastics:
- The theory of brittle fracture states that cracks form at the site of greatest stresses and gradually increase. When the critical length is reached, division into parts begins. Before the formation of cracks, the body completely obeys Hooke's law( a force proportional to elongation).The fracture stress is described and the formula depends on the specific energy of destruction of the material. Lack of theory: before the formation of cracks, thermoplastics begin to deform, expending energy.
- Thermofluctuation theory of strength speaks of a quantitative relationship between the applied stress and the time that passes before failure. These parameters are connected by an exponential formula, which in addition includes two constants( see figure).Zhurkov equation is more complicated and takes into account the activation energy of destruction. The thermofluctuation theory asserts that destruction becomes a kinetic process of accumulation of damage, and not a one-time act. In the course of the phenomenon cracks are formed.
The latest theories cast aside the structure of polymers, which is recognized as a disadvantage. It does not take into account the physical condition. Most of the data obtained predominantly empirically. For example, the behavior of thermoplastics under short-term load is described by the graphs obtained in experiments. Then the curves find the values:
- The short-term modulus of elasticity is determined from the angle of inclination of the tangent, drawn from the origin of the curve for a low loading rate. A secant modulus of elasticity is found by the angle of inclination of the secant of the previous graph.
- Breaking stress. The graph is marked with a cross at the end of the curve. Determined for polymers that break down brittle.
- Yield Strength. Analogs of breaking stress for viscous polymers. The largest indicators of this and the previous parameter in polymers of group I, the lowest - in III.
- Energy of destruction. Numerically equal to the area under the curve. In case of high-speed destruction, work is evaluated.
- The brittleness temperature is estimated from curve families. The nature of the damage is evaluated under various conditions( determined by the shape of the curve).According to GOST 16782, the sample is loaded at a constant speed( from 4.5 to 120 m / min) with a simultaneous temperature change from experience to experience. Record the environmental indicators at which the destruction occurs.
plots Other parameters:
- The standard hardness is determined according to Brinell and characterizes the resistance to spherical indenter penetration.
- Standard heat resistance describes the temperature at which the deformation exceeds the limit values. The determined figures strongly depend on the methods: double-support bending, Martens bending, the introduction of the Vic's cylindrical needle.
- Poisson's Ratio shows the change in volume during deformation. It depends on temperature, strain rate and its magnitude. Maximum values for group III thermoplastics.
- Impact strength is determined by the relatively slow destruction of the specimen at a temperature of 20 degrees Celsius by the impact of the copra during double-support bending( GOST 4647).Sharply decreases with the appearance of cuts, strongly depends on the shape and depth of damage. Specific values are highly dependent on the technique.
- Impact toughness allows us to estimate the strength under high-speed loading. Polymers of groups II and III are characterized by the highest values, the lowest indicators for representatives of group I are polystyrene and polymethylmethacrylate. In PVC, the parameter is high at a temperature of +20 degrees Celsius, drops sharply when cooling.
Temperature and rate of loading have a perceptible effect on the shape of the graph. However, uniform dependence is not observed. The similarity of the processes is observed within groups, previously characterized by physical structure. Characteristics are highly dependent on the process. For example, during the annealing of polymers of group I near the glass transition temperature, the elastic modulus increases. After an hour and a half of PVC exposure at a temperature of 60 degrees Celsius, the 10-second modulus of elasticity is 160 kgf / sq.mm, after 48 hours - 230, after 60000 hours - 270.
The maximum variation of the modulus of elasticity and hardness in the third group. Test methods for thermoplastics are far from perfect, but heat shrink tubing is used in everyday life and industry. The question is close to electricians. Actually, the subject of patent US3396460 A was developed for them. Heat-shrinkable films are used to protect control panels, polymers are used to pack products.